The invention provides new methods for purifying and concentrating
viruses. The inventors have discovered that high molecular weight
proteoglycans present in retroviral stocks are co-concentrated with the
retroviruses, and can inhibit retroviral transduction. The new
purification and concentration methods feature treatment of virus stock
with an anionic polyelectrolyte and a cationic polyelectrolyte, followed
by centrifugation. The new methods minimize the amount of proteoglycan
co-precipitated with the infectious virus.